The present invention is directed to an electric motor-actuated load clamp for automatically-guided vehicles, and particularly to such a load clamp having accurate clamping force control and a high degree of reliablity and durability.
Most materials handling clamps, of the type for handling cartons, paper rolls and the like, are actuated by fluid power actuators under the control of valves which regulate the supply of pressurized fluid to the actuators. In such load-handling clamps it is important to apply enough clamping force to the load to ensure that it is not inadvertently released from the frictional grasp of the clamp arms, and yet insufficient clamping force to damage the load. The optimum clamping force to satisfy both objectives differs with the type of load, and therefore it is common for the control valves of fluid power clamps to have multiple settings to provide a plurality of different fluid pressures and thus different clamping forces.
Such fluid power actuators are not well adapted for clamp arm actuation when used with automatically-guided, driverless vehicles because of the expense and difficulty of interfacing them with a microprocessor so as to achieve the high accuracy of clamp arm actuation required for an automatic unit which lacks a human operator. Electrically-actuated load clamps are better adapted to this application, such as the electrically-actuated clamp disclosed in the commonly-owned U.S. patent application of one of the inventors herein, Ser. No. 677,929, filed Dec. 4, 1984.
Electrically-actuated load clamps in general are commonly used in many applications as evidenced, for example, by the clamps shown in U.S. Pat. Nos. 2,959,445 and 3,815,761, the latter including a current limiting device to limit the motor torque and thereby limit the gripping force exerted by the clamp. It is also known to sense the current level in an electric motor, or the electromagnetic field resulting therefrom, and to control the current level within a maximum and minimum range in response thereto as suggested, for example, in U.S. Pat. No. 3,824,440. However, such systems, if applied to a load clamp for an automatically-guided vehicle, would not enable the precise clamping force control necessary to ensure reliable load gripping while preventing overgripping, nor would they enable the accurate automatic selection of any of a plurality of different predetermined clamping forces for different types of loads. Moreover, such systems are not able to interrupt power to the motors automatically to prevent overheating during load clamping, while nevertheless ensuring continuity of adequate clamping force without overgripping throughout an extended period of time.
Related problems peculiar to an automatic clamp system, not dealt with by the prior art, include ensuring that the load is adequately gripped if there should be a malfunction in the clamp open/close control system, or a lack of uniform distribution of electric current among a plurality of clamping motors. Another problem for which a solution is needed is how to compensate for the effect on clamping force of transient current in-rush when the clamping motors are started.